1. Field of the Invention
The present invention relates to an ultraviolet (UV) light ionisable gas detection device and method of enhanced performance which is suitable for environmental pollution monitoring, atmospheric tracer detection and monitoring gas or vapour emitting processes.
2. Discussion of Prior Art
Ultraviolet light ionisable gas detectors have application in a variety of fields. For example, atmospheric tracer techniques are used for various meteorological and environmental pollution studies where short range diffusion phenomena are under investigation. One known technique involves the release of a tracer compound into the atmosphere and the detection of this at a distant point by use of an ultraviolet light exciter device. In such devices tracer is excited by influence of UV light to produce charged species which are subsequently detected by use of a voltage bias electrode unit.
Typical UV ionisable tracer compounds are olefins, ketones or aldehydes which are capable of taking gas form under standard pressure and temperature conditions, but any volatile or gaseous organic compounds which yield charged materials under influence of ultraviolet light may be used. Typical gaseous examples for tracer use are propylene or butylene.
In a further application of such devices known to the applicant, use of UV light sources of selected wavelength and energy-level makes it is possible to ionise some molecules and not others and thus gives selective detectabililty. For example, comparing the ionisation potential of various compounds, water (12.59 eV), ethane (11.65 eV), ammonia (10.15 eV), nitrobenzene (9.82 eV) and styrene (8.47 eV), it is clearly possible to monitor release of certain compounds against a background of others by selection of UV source wavelength. Such capability clearly has application in a variety of monitoring areas such as in environmental pollution monitoring for release of gases and vapours or monitoring of laboratory processes where gases and vapours are emitted.
One known UV exciter/detector device is described in GB 1576474 and comprises a collector electrode in the form of a washer-like annulus less than 3 mm high and of less than 1 mm material thickness mounted on the gas passage wall and extending across the path of UV radiation emitted from a source behind the electrode. This device has a bias electrode with its distal end central of the annulus and directly exposed to the UV radiation with the collector electrode shielded from the UV radiation by opaque plastics or metallic material.
The collector electrode and bias electrode lie with the major dimensions of their adjacent portions in planes transverse to each other and are adjacent only at the end of the bias electrode. Incoming gas or air for analysis is drawn across the electrode assembly where it is irradiated by UV radiation passing through a window in the shield. This device has a gap between the two electrodes of about 4 mm and the patent specification for this device states that it provides increased sensitivity and linearity over parallel plate electrode devices.
A second known improved UV exciter/detector device also uses a washer like annular electrode upstream of a UV radiation source but in this case an insulator is provided between it and an oppositely chargeable wire mesh electrode. The electrode assembly lies with its major dimension placed directly across the gas flow path and the UV radiation is directed onto the wire mesh electrode such that gas flowing through the system passes directly onto the UV source after passing through the electrode arrangement. This device has a gap of 2 mm between the electrodes.
Whereas the first device has only the bias electrode lying with its major dimension across the flow path, this second device has both electrodes so arranged. In both devices it is desirable to include a dust filter for removing particulates from gases entering as both arrangements are prone to build up of deposits on the UV source and/or electrodes and thus require regular cleaning. Furthermore when used with a olefinic tracer these arrangements are vulnerable to build up of polymers on the surface emitting UV light, this resulting in a polypropylene film where propylene is used.
This second device only operates satisfactorily with an intake air flow of up to 8.33.times.10.sup.-6 cubic meters sec.sup.-1 which necessarily limits sensitivity to tracer or other vapour by setting a limit upon the size of sample that can be investigated per unit time. Furthermore the responsiveness of this known device to rapid fluctuation of gas concentration is also limited, typically to about 1 Hz, a feature which is unsatisfactory from a pollution monitoring stand-point or in monitoring fast changing levels of a product or analyte. One particular application for such UV exciter/detector devices has emerged in the field of screening containers for contaminants prior to their reuse. Obviously the ability only to detect fluctuations of 1 Hz renders such screening limited to one container every few seconds if any degree of accuracy is to be ensured. These drawbacks make it desirable to provide a more sensitive and responsive device than that hitherto available.